article: Did you notice the number on the book in a library? That number is part of the system used by libraries to organize their collections of books. And it's used in many countries. The number on each book tells you exactly what kind of book it is. This system is also useful for knowing where to go in the library to find a book.
  In this system, there are ten large groups of books. Each of these groups has its own number, such as 100, 200, etc. So, for example, any books about language will have a number 400. On the other hand, any books about history will have a number 900. So, a number in the hundreds place tells you what general group a book is in. If you find a book that has a number in the 500s, you know it is a book about science.
  However, science is a big group, so the tens place is used to make a more detailed set of science books. For example, math books are included in the group of science books. Math books all have numbers between 510 and 519. Books about the history of Africa have numbers between 960 and 969.
  The system uses the ones place to give a more exact limit for the subject of a book. A book on the history of South Africa will have the number 968.
  As you can see, it is a simple system to use as long as you understand what the numbers mean. With this system, the library can keep its books well organized, and people can easily find the book that they want.
question: The reading is about  _  .
options: libraries working in a library how numbers are organized how books are organized in libraries